When a semiconductor integrated circuit for radio communication is used in an apparatus for mobile communication, power consumption needs to be minimized for prolonged battery life. To achieve this, power source control is performed so that the operation of a circuit involved in a function that is not currently used during the call waiting state or the like, is stopped. For example, JP H06-232349A describes power source control that is performed to operate/stop each functional block, but not individual circuits separately. In other words, a different power source control is performed for each functional block in order to stop the supply of power to a block that is not currently used so as to stop the operation of the block.
In the above-described technology using the power source control system, however, operation/stop is performed for each functional block comprising a set of circuits. Among individual circuits contained in a functional block, power is supplied to a circuit(s) not in use when the corresponding functional block is operated, so that current is wastefully consumed.
For example, only a voice signal power amplifier is required to be operated when outputting a transmission data signal to a loudspeaker. However, when power source control is performed on the each functional block basis, it is required to operate a whole block for processing a receiving voice band signal, resulting in wasteful current consumption. Furthermore, when a semiconductor integrated circuit for radio communication having a voice signal level detector is used, even in a radio communication apparatus that does not use a voice signal level detector, it is required to operate a whole block for processing a transmission voice band signal, resulting in wasteful current consumption. Also, there is a problem with a noise detector that is used in combination with an intermediate frequency detector for the purpose of detecting the level of a receiving signal. That is, taking into account the stage of detecting the receiving signal level, the operation of the noise detector is effective only when the receiving signal level can be detected by the intermediate frequency detector placed near a receiving signal input terminal. However, when power source control is performed on the each functional block basis, it is required to operate a whole block for processing a receiving high frequency signal, resulting in wasteful current consumption.
On the other hand, when power source control is performed on the each functional block basis only for the purpose of reducing current consumption, load fluctuation associated with the operation/stop of an element circuit has an influence on another element circuit connected thereto in the same functional block, resulting in deterioration of the rising characteristics of a power source.
FIG. 8 shows the output frequency characteristics of a local oscillator in a conventional semiconductor integrated circuit for radio communication. In this example, a multiplier for generating an intermediate frequency signal based on a receiving signal and a local frequency signal and a local oscillator are subjected as a receiving-system high frequency block to the same power source control. FIG. 8 shows a frequency change of a local frequency signal output from the local oscillator when the multiplier and the local oscillator start operating simultaneously. t1 represents the operation start time of a receiving-system high frequency block and t2 represents a time at which the frequency of the local oscillator becomes stable. Δt represents an interval between t1 and t2, indicating an elapsed time from the start of operation of the receiving-system high frequency block until the frequency of the local oscillator becomes stable. The output of the local oscillator is coupled to the input of the multiplier. Therefore, when the local oscillator and the multiplier start operating simultaneously, the local oscillator is operated before the input impedance of the multiplier becomes stable. For this reason, it takes a long time for the frequency of the local oscillator to become stable.